The present invention relates to an exhaust gas purifying apparatus for an internal-combustion engine and, more specifically, to an exhaust gas purifying apparatus for an internal-combustion engine that can accurately measure NOx trapping capability of a nitrogen oxide (NOx) catalyst.
Conventionally, there is a known technique for decreasing NOx in the exhaust gas by a NOx purifying device that is located in the exhaust system of the internal-combustion engine and incorporates a NOx trapping agent. Japanese Patent Application Unexamined Publication (Kokai) No. H10-299460 discloses a technique for determining deterioration of the NOx purifying device based on delay time of outputs of oxygen density sensors disposed upstream and downstream respectively of the NOx purifying device when the fuel control air-fuel ratio of the engine is set richer than a stoichiometric air-fuel ratio after lean-burn operation is made for a given time period. More specifically, in the conventional technique, deterioration of the NOx purifying device was determined based on a fact that the delay from the time the output of the upstream oxygen density sensor changed to rich to the time the output of the downstream oxygen density sensor changes to rich is relatively long when the NOx trapping capability of the NOx purifying device is high whereas such delay time becomes shorter as the NOx trapping capability degrades.
However, in such conventional techniques, there exists a problem that deterioration of the NOx purifying device cannot be accurately determined because the time period from the time the upstream oxygen density sensor changes to a rich state to the time the downstream oxygen density sensor detects a rich state varies as the air fuel ratio of the gas flowing into the NOx purifier varies at different speeds. Such variation is due to unstable air-fuel ratio of the exhaust gas flowing into the NOx purifying device, which may be caused by the influence of such factors as 1) variation of deterioration of the three-way catalyst disposed upstream of the NOx purifier, 2) variation of activation degree of the three-way catalyst disposed upstream of the NOx purifier and 3) variation of sulfur constituent contained in the fuel.
Thus, it is an objective of the present invention to provide an exhaust gas purifying apparatus that can accurately measure a NOx trapping capability of the NOx purifying device.
According to one aspect of the present invention, an exhaust gas purifying apparatus for an internal-combustion engine is provided. The exhaust gas purifying apparatus comprises a three-way catalyst that is disposed in an exhaust system of the internal-combustion engine and a nitrogen oxide purifier that is disposed downstream of the three-way catalyst for purifying or cleaning nitrogen oxide that is contained in the exhaust gas when an air fuel ratio of the exhaust gas of the engine is lean. The purifying apparatus further comprises an upstream oxygen sensor that is disposed between the three-way catalyst and the nitrogen oxide purifier, and a downstream oxygen sensor that is disposed downstream of the nitrogen oxide purifier. The purifying apparatus also comprises a correcting means for correcting a criterion for determining abnormality of the nitrogen oxide purifier. The criterion is used for determining abnormality of the NOx purifier based on a change in the output of the downstream oxygen density sensor when the air-fuel ratio of the exhaust gas is changed from lean to rich. Correction of the criterion is made according to the degree of change in the output of the upstream oxygen density sensor.
According to the invention, means is provided for correcting the criterion for determining abnormality of the nitrogen oxide purifier. The criterion is the delay time that the output of the downstream oxygen sensor changes from lean to rich after the output of the upstream oxygen sensor changes from lean to rich. The criterion is corrected in relation to the speed the output of the upstream oxygen sensor changes from lean to rich or the time the upstream oxygen sensor takes to change from lean to rich. This way, the influence of the upstream three-way catalyst on the oxygen density sensors can be excluded, and deterioration of the nitrogen oxide purifier can be accurately determined. In one aspect of the invention, correction of the criterion may include correction of reversal delay time of the downstream oxygen density sensor (that is, the delay time for the downstream oxygen density sensor to change from lean to rich after the upstream oxygen density sensor changes from lean to rich). Correction of the criterion may also include correction of thresholds for determination.
According to another aspect of the present invention, in the exhaust gas purifying apparatus for the engine, the abnormality determination criterion is defined in terms of a delay time from the time the output of the upstream oxygen density sensor reverses as the air fuel ratio is changed from lean to rich to the time the output of the downstream oxygen density sensor reverses. The nitrogen oxide purifier is determined abnormal when the output of the downstream oxygen density sensor reverses before the time period reaches the criterion.
According to one aspect of the invention, the influence of the upstream three-way catalyst upon the oxygen sensors can be eliminated. In one embodiment of the invention, the criterion is modified based on a cumulative intake air amount value GSLFFIN that is an air amount taken-in from the time the upstream oxygen sensor output SVO2 exceeds a first upstream reference value SVO2LNCS, which indicates that SVO2 started to rise, till the time SVO2 reaches a second upstream reference value SVO2SLF, which SVO2 would reach within a reasonable time if SOx density is low. When the engine revolution is stable, GSLFFIN corresponds to the time the upstream oxygen sensor took to reverse from a lean state to a rich state. In other words, GSLFFIN corresponds to the speed the upstream oxygen sensor responds to changing air fuel ratio.